The present invention relates to a solid-state imaging element such as, e.g., a CMOS image sensor and a method for fabricating the same. More particularly, the present invention relates to a solid-state imaging element comprising an antireflection film on a photodiode formed in a semiconductor substrate and a method for fabricating the same.
As a solid-state imaging element, a CMOS image sensor comprising a pixel portion and a peripheral CMOS logic circuit portion has been known. The CMOS image sensor has such advantages as a high quantum efficiency, a high dynamic range, and a random access so that it is easy to provide compatibility between the fabrication process thereof and a CMOS process. As a result, it is possible to form an A/D converter and various signal processing circuits in the same chip. In such a solid-state imaging element, trends toward a larger number of pixels and a smaller pixel size have been rapidly growing in recent years. As a result, it has become a significant challenge to maintain or improve the sensitivity.
In the solid-state imaging element, a photodiode is formed as a light receiving portion in a semiconductor substrate. Due to the refractivity difference between an interlayer insulating film (which is typically a silicon dioxide film) formed on the photodiode and silicon composing the semiconductor substrate, a part of incident light is reflected upward by the surface of the semiconductor substrate. As a result, the light reaching the photodiode decreases to result in degradation of the sensitivity. To prevent this, it has been known to, e.g., provide an antireflection film made of a silicon nitride film on a silicon substrate via a silicon dioxide film, thereby reducing a loss in incident light using a multiple interference effect, and improving the sensitivity.
As an example of a method for fabricating a CMOS image sensor, a method has been proposed which forms an antireflection film on a photodiode, while maintaining compatibility with a CMOS process, as will be described hereinbelow (see, e.g., U.S. Pat. No. 6,906,364 B2).
FIGS. 11A to 11E are principal-portion cross-sectional views sequentially showing the process steps of the conventional method for fabricating the solid-state imaging element disclosed in the U.S. patent mentioned above.
First, as shown in FIG. 11A, an isolation region 101 made of a silicon dioxide film is formed in a semiconductor substrate 100 having a photodiode region 100A composing a pixel portion, and a transistor region 100B composing a CMOS logic circuit portion. Subsequently, a silicon dioxide film 106 is formed by thermal oxidation on the semiconductor substrate 100, and then a polysilicon film 108 is formed by a reduced pressure CVD method.
Next, as shown in FIG. 11B, the silicon dioxide film 106 and the polysilicon film 108 are patterned by photolithographic and etching techniques to form a gate oxide film 106a and a gate electrode 108a. Subsequently, ion implantation 110 is performed to form a lightly doped impurity diffusion layer 112 in the area of the semiconductor substrate 100 located laterally and outwardly under the gate electrode 108a using the gate electrode 108a as a mask in the transistor region 100B, while simultaneously forming a lightly doped impurity diffusion layer 112 in the upper portion of the semiconductor substrate 100 in the photodiode region 100A.
Next, as shown in FIG. 11C, a silicon dioxide film or a silicon nitride film is deposited over the entire surface of the semiconductor substrate 100 using a reduced pressure CVD method, and then subjected to anisotropic etching to form sidewall spacers 114 on the side surfaces of gate electrode 108a. Subsequently, ion implantation 118 is performed to form source/drain diffusion layers 120 in the areas of the semiconductor substrate 100 located laterally and outwardly under the sidewall spacers 114 in the transistor region 100B, while simultaneously forming a heavily doped impurity diffusion layer 122 in the photodiode region 100A.
Next, as shown in FIG. 11D, a silicide formation preventing film 124 made of a silicon dioxide film is formed on the isolation region 101, and on the heavily doped impurity diffusion layer 122 in the photodiode region 100A. Subsequently, a silicide layer 126 is formed on each of the upper surfaces of the gate electrode 108a and the source/drain diffusion layers 120 by a salicidation method.
Next, as shown in FIG. 11E, a liner layer 128 made of a silicon nitride film is formed over the entire surface of the semiconductor substrate 100 including the photodiode region 100A and the transistor region 100B using a plasma CVD method. At this time, in the photodiode region 100A, the liner layer 128 made of the silicon nitride film has a refractivity different from that of the silicide formation preventing film 124 made of the silicon dioxide film, and functions as the antireflection film.
In the conventional method for fabricating the solid-state imaging element described above, the silicon dioxide film or the silicon nitride film composing each of the sidewall spacers 114 of the transistor is removed in the photodiode region 100A by dry etching during the formation of the sidewall spacers 114, as shown in FIG. 11C. As a result, during the dry etching in the process step, the surface roughening of the semiconductor substrate or the unexpected entrance of a metal impurity into the semiconductor substrate occurs at the surface of the photodiode to cause the problem of the degraded characteristics (a lower sensitivity, increased sensitivity variations, and an increased dark current) of the image sensor.
As shown in FIG. 11E, the liner layer 128 made of the silicon nitride film is used as the antireflection film in the photodiode region 100A. However, it is difficult in terms of fabrication to cause the film thickness (about 20 to 30 nm) required of the liner layer functioning as an etching stopper during, e.g., the formation of a contact hole in a CMOS fabrication process to coincide with the optimum film thickness (about 30 to 80 nm) required of the liner layer functioning as the antireflection film. This also leads to the problem that a sufficient antireflection effect cannot be obtained in the photodiode region 100A.